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Sclerotinia Resistance: Real Progress

Wednesday, April 2, 2008
filed under: Disease

The first edition of North Dakota State University’s Sunflower Production handbook, published in 1975, had this to say regarding the control of Sclerotinia:

“Resistant varieties are unknown, and the chances of developing such are not likely. Fungicides which will control the disease are not yet approved for sunflowers. Consequently, losses must be minimized by managerial practices. The disease is one of the most difficult to control in the field.”

Though not a very encouraging snapshot, those 1975 statements were, unfortunately, pretty accurate. Until recently.

The good news is that after literally decades of wrestling with this vexing multi-gene disease, substantial progress is now occurring in the effort to develop commercial sunflower hybrids with good resistance to Sclerotinia. The welcome progress of recent years is due to additional research funding through the National Sclerotinia Initiative, an unprecedented interdisciplinary approach by USDA scientists, the employment of biotechnology, improved screening techniques, strong cooperation between the public and private sectors — and just plain perseverance by all involved.

Where has that mixture brought us?

“If we were to compare the hybrids of the 1980s — and even the 1990s — for stalk rot resistance, they would be uniformly susceptible,” says USDA-ARS research plant pathologist Tom Gulya. “Within the past five years, we have shown that many commercial hybrids now have significant levels of stalk rot resistance (but not immunity) — and a few hybrids have resistance to both the stalk and head rot versions of Sclerotinia. We could not have said that 10 or even five years ago.”

The 2007 USDA Sclerotinia stalk rot trials provided a graphic example of how far we have come. The trials encompassed three locations and 97 hybrids (all artificially inoculated with Sclerotinia) from 13 companies. Measured at maturity (late September) and averaged across locations, stalk rot levels ranged from 3% on the low end up to about 63% on the high side.

The resistant check — a hybrid from USDA inbreds developed specifically for stalk rot resistance — averaged 12% infection. Two years ago, the USDA hybrid was the best performer; no commercial hybrid equaled its level of stalk rot resistance. In 2007, however, 24 of the entered hybrids were equal to or better than the check — a clear indication of the progress made by seed companies in developing hybrids with significantly better stalk rot resistance.

Much of this decade’s progress is due to more-efficient screening. Resistance to Sclerotinia has always existed; it’s basically been a matter of (1) finding it and (2) being able to incorporate it into hybrids without sacrificing important agronomic traits and seed/oil quality characteristics.

That sounds much easier than it is. There’s the “needle” (good resistance) and the “haystack” (sunflower has 51 species and subspecies, a small minority of which are represented in cultivated parentage; the rest is found in annual and perennial wilds).

Once a promising level of resistance is found in a given population, that resistance must — through a very complicated, time-consuming process — be placed into elite germplasm for hybrid development. It’s a daunting task — particularly with perennials. Toss in the fact that (1) Sclerotinia resistance is a multi-gene trait (as opposed to the single-gene nature of rust or downy mildew resistance), and (2) stalk rot and head rot each require a different set of resistance genes — and it’s easier to understand why “progress” has been such a long and winding road.

The National Sclerotinia Initiative is a coordinated research effort whose objective is to reduce the economic threat of Sclerotinia in seven crops: sunflower, soybeans, canola, dry edible beans, chickpeas, lentils and dry peas. Coordinated by the USDA’s Agricultural Research Service, the consortium includes federal and state scientists, land grant universities and crop commodity groups, including the National Sunflower Association.

The degree of cooperative research within the sunflower sector has caught the attention of everyone involved in the national Sclerotinia effort. The USDA-ARS group — the majority of whom are based at Fargo, N.D. — includes:

Gerald Seiler — A botanist, Seiler has long focused on the collection and categorization of wild species populations for the purpose of evaluating and, hopefully, incorporating their beneficial traits into cultivated sunflower. He has traveled throughout the United States and several foreign countries. A special challenge with collecting wilds for Sclerotinia resistance is that this disease does not infect the wilds (unlike rust, for example). So one must collect as many populations as possible, from diverse species and widespread locations, and then evaluate them in greenhouse and field tests to determine their level of Sclerotinia resistance.

Laura Marek — Marek is curator of the sunflower collection at the ARS Plant Introduction Station in Ames, Iowa. Its wild sunflower collection presently contains 2,136 annual and perennial accessions. One of Marek’s (and Seiler’s) primary responsibilities is to increase populations of “under-represented” species so that sufficient seed is available to USDA and other scientists for testing. (The Ames sunflower collection also includes 1,702 cultivated accessions.)

Charlie Block and Tom Gulya — Block is an Ames-based ARS plant pathologist; Gulya is, as noted earlier, a pathologist with the ARS unit at Fargo. They take the Ames seed accessions and test them in the greenhouse for stalk rot resistance. Wild species appearing to be resistant are then evaluated further in field trials.

C.C. Jan — Once a wild species is confirmed to have stalk rot resistance, the plants are turned over to Jan, a Fargo-based ARS cytogeneticist whose specialty is in crossing wild species with cultivated sunflower. Since many wild species have two or three times the chromosome number of cultivated sunflower, special techniques (and much patience) are required to successfully accomplish such crosses. Also, many generations of crosses are required to transfer the genes into a plant that closely resembles cultivated sunflower.

Brent Hulke — After the wild species have been crossed with cultivated lines and the chromosome number reduced to diploid status, the material goes to Hulke. A Fargo ARS research geneticist who succeeded longtime USDA breeder Jerry Miller in 2007, he must ensure that the finished breeding line has not only Sclerotinia resistance, but also the necessary traits of high yield potential, high oil content, etc.

Hulke additionally works with molecular markers for the identification of Sclerotinia resistance. One of the newer “high tech” aspects of plant breeding, this laboratory method of visualizing resistance genes has thus far resulted in at least 16 markers being identified for head rot resistance and another six markers for stalk rot resistance. The marker approach allows researchers to gain some idea of a plant’s level of resistance early on in the breeding process — and long before the plant reaches maturity. This permits researchers to “pre-select” plants with higher-than-average resistance, thus reducing the number of poor performers that get tested in the field.

State Universities & Private Sector — Key cooperators conduct field tests on both head and stalk rot breeding lines and hybrids. Sites are located at Carrington, Oakes and Langdon, N.D., as well as Crookston, Minn., and Morden, Manitoba.

Bringing a population from step one (identification of resistance) to the final step (releasing an inbred line to commercial breeders) can take upwards of eight or nine years. Fortunately, promising materials have been moving through the ARS pipeline for some time now, so it’s not as if sunflower growers must wait another nine or 10 years to actually have Sclerotinia-resistant hybrids in their fields. Indeed, as reflected in the chart on page 17, more and more commercial hybrids are already displaying notably better resistance — and that’s only going to get better and more commonplace as each year passes.

Will we ever have sunflower hybrids that are totally immune to Sclerotinia? Never say never, but that’s probably an unrealistic goal for the near future, according to Gulya. “Since Sclerotinia resistance is controlled by so many genes, our goal is to find and add them — one by one,” he says. “With each additional gene, we’ll achieve a higher level of resistance.

“If the industry can develop a hybrid that consistently gets only 5% infection, our goal can be to bring that down to 2%. But realistically, 5% for a given hybrid — compared to one 10 years ago that probably would have been at 55% — is a huge improvement. We can live with a very low level like that.” — Don Lilleboe
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